An X-ray is an electromagnetic wave having a high energy which has a power to penetrate through many materials, e.g.- a human body, packed materials, industrial products and the like, and a transmitted X-ray has a space distribution in its intensity due to the difference in the absorption coefficients in the materials. If our eyes had the ability to see X-rays, we would be able to see the space distribution of the transmitted X-rays, i.e.- the inside of the materials. Unfortunately, our eyes cannot detect X-rays. To observe the inside of the materials, a conversion from X-rays to another media, which can be observed with our eyes, is absolutely necessary. However, there is no available three dimensional image detector, and a transmitted X-ray is usually projected on a two dimensional plane (e.g.- a screen), which detects X-rays, so as to observe the inside of the materials. A typical X-ray detecting screen is an X-ray film which is coated with an X-ray sensitive emulsion of a suspended fine powder of silver halides. When the X-ray film has been exposed to X-rays, a latent X-ray image is formed in the X-ray film and the exposed film is chemically developed in a darkroom to semi-permanently fix the X-ray image on the X-ray film. The developed X-ray film has different reflections (or transmittances) with respect to light, e.g.- dark portions correspond to exposed areas and light portions correspond to unexposed areas. Thus, doctors in hospitals can diagnose the inside of a body by observing the developed X-ray film under a light or by transmitted light, and the inspectors in industrial processes and security areas can nondestructively inspect the inside of materials of industrial products and packages. Thus, the X-ray images are essential for enabling doctors in hospitals and clinics to diagnose patients, and for enabling inspectors to inspect the products in quality control areas of industry, and for enabling security guards to inspect packages in security areas.
Because X-ray film has a poor sensitivity to X-ray exposure, a large dosage of X-rays to a patient's body is needed for the formation of a proper X-ray image on the X-ray film for use in diagnostic purposes by doctors. The large dosage of X-rays to a body is hazardous and sometimes causes severe damage to the health of patients. The reduction of the dosage of the exposure of X-rays to the human body is a primary subject for radiologists. A successful approach is an application of a fluorescent intensifier screen which converts X-rays to a fluorescent light in the wavelengths of the maximum senstivity of X-ray film, noting that X-ray film is more sensitive in the near ultraviolet light range in comparison to X-rays. If the fluorescent intensifier screen is attached to the X-ray film, then the X-ray film absorbs both X-rays and fluorescent light; naturally, the formation of the X-ray image on the X-ray film is significantly improved. The fluorescent intensifier screen is coated with phosphor crystals which have a high absorption coefficient of X-rays and a high conversion efficiency of luminescence. However, the application of the fluorescence intensifier screen does not eliminate the essential difficulties of the use of X-ray film. They are: the need for a chemical process in a darkroom; the need for a treatment of the image information to enhance the objective images on the X-ray film; and storage of the developed X-ray films.
The X-ray image intensifier tube, i.e.- a special cathode ray tube consisting of a layer that emits photoelectrons as X-rays are absorbed, and electrodes for accelerating the photoelectrons, and electric lenses and a phosphor screen which emits cathodoluminescence when the accelerated photoelectrons hit the phosphor screen, has been developed to eliminate the need for a process in a darkroom. The X-ray image intensifier tube detects a small limited area of the body, and the X-ray image on the phosphor screen is only observed during the exposure of X-rays to the body. This increases the X-ray exposure dosage on the body which is needed for diagnosis by doctors. The permanent or semi-permanent recording of the X-ray image on a screen may therefore reduce the X-ray exposure dosage on the body.
There has been an attempt to record the X-ray image on the phosphor screen, that is, a phosphor screen utilizing thermoluminescence. Some phosphors emit thermoluminescence if the phosphors which are exposed to X-ray radiation are heated to an elevated temperature by either irradiation by an infrared light or by a laser beam which is focused on the exposed screen. However, the sensitivity to X-rays and intensity of thermoluminescence are not high enough to obtain a sharp image. A more practical device utilizes an electrostatic image formed on the dielectric layer. The dielectric layer is charged as the electric field has been applied, and the charged layer is discharged as X-rays irradiate the charged layer. The sensitivity of such a device is not high enough for a wide application to radiology, limiting the application of such a device.